1. Field of the Invention
This invention relates generally to the field of treating wells to stimulate fluid production. More particularly, the invention relates to the field of abrasive jet perforating of wellbore casings.
2. Description of the Related Art
Abrasive jet perforating uses fluid slurry pumped under high pressure to perforate casing and cement around a wellbore and to extend a cavity into the surrounding reservoir to stimulate fluid production. Since sand is the most common abrasive used, this technique is also known as sand jet perforating (SJP). Sand laden fluids were first used to cut well casing in 1939. Abrasive jet perforating was eventually attempted on a commercial scale in the 1960s. While abrasive jet perforating was a technical success (over 5,000 wells were treated), it was not an economic success. The tool life in abrasive jet perforating was measured in only minutes and fluid pressures high enough to cut casing were difficult to maintain with pumps available at the time. A competing technology, explosive shape charge perforators, emerged at this time and offered less expensive perforating options.
Consequently, very little work was performed with abrasive jet perforating technology until the late 1990's. Then, more abrasive-resistant materials used in the construction of the perforating tools and jet orifices provided longer tool life, measured in hours or days instead of minutes. Also, advancements in pump materials and technology enabled pumps to handle the abrasive fluids under high pressures for longer periods of time. The combination of these advances made the abrasive jet perforating process more cost effective. Additionally, the recent use of coiled tubing to convey the abrasive jet perforating tool down a wellbore has led to reduced run time at greater depth. Further, abrasive jet perforating did not require explosives and thus avoids the accompanying danger involved in the storage, transport, and use of explosives. However, the basic design of abrasive jet perforating tools used today has not changed significantly from those used in the 1960's.
Abrasive jet perforating tools were initially designed and built in the 1960's. There were many variables involved in the design of these tools. Some tool designs varied the number of jet locations on the tool body, from as few as two jets to as many as 12 jets. The tool designs also varied the placement of those jets, such, for example, positioning two opposing jets spaced 180° apart on the same horizontal plane, three jets spaced 120° apart on the same horizontal plane, or three jets offset vertically by 30°. Other tool designs manipulated the jet by orienting it at an angle other than perpendicular to the casing or by allowing the jet to move toward the casing when fluid pressure was applied to the tool.
Occasionally, a tool employed a centralizer to keep the tool from touching the low side of the casing. Conventional tools typically have a uniform outer diameter, with the exception of the mounting locations for the jets. Mechanical casing collar locators generally consisted of a tool with a hollow shaft for fluid travel, and a “slip” (or “dog”) that resides in a pocket on the outside of the tool and is pressed against the casing by a spring located in between the pocket and the slip.
The following patents are representative of conventional abrasive jet perforating tools, along with apparatus and methods that may be employed with the tools.
U.S. Pat. No. 3,130,786 by Brown et al., “Perforating Apparatus”, discloses an abrasive jet perforating tool. The tool comprises a cylindrical conduit for abrasive fluid to be pumped through and jet nozzles laterally extending from the conduit to direct the abrasive fluid through the casing into the surrounding formation. Factors such as the pressure differential and the ratios of the diameter of the nozzle orifice to the length of the nozzles and to the size of the abrasives are kept within predetermined limits for optimum penetration.
U.S. Pat. No. 3,145,776 by Pittman, “Hydra-Jet Tool”, discloses protective plates for an abrasive jet perforating tool. The plates, made of abrasive resistant material, are designed to fit flatly to the body of the tool around the perforating jets. The plates are employed to protect the body of the tool from ejected abrasive material that rebounds. The protective plates disclosed in Pittman are not designed to protect the abrasive jets themselves.
U.S. Pat. No. 3,266,571 by St. John et al., “Casing Slotting” discloses an abrasive jet perforating tool designed to cut slots of controlled length. The slot lengths are controlled by abrasive resistant shields attached to the tool to block the flow from rotating abrasive jets.
U.S. Pat. No. 3,902,361 by Watson, “Collar Locator” discloses a mechanical casing collar locator that can be used with, among other tools, an abrasive jet perforating tool. A spring-loaded tagging element engages the annular shoulder formed between the spaced ends of adjacent casing joints joined together by the collars. A tubing weight indicator senses each time a collar is located.
U.S. Pat. No. 4,050,539 by Tagirov et al., “Apparatus for Treating Rock Surrounding a Wellbore”, discloses an abrasive jet tool for successively perforating and then fracturing reservoirs. The nozzles of the abrasive jets are designed to snugly fit against the casing to allow perforating at one pressure immediately followed by fracturing at a higher pressure.
U.S. Pat. No. 5,499,678 by Surjaatmadja et al., “Coplanar Angular Jetting Head for Well Perforating”, discloses a jetting head for use in an abrasive jet perforating tool. The jet openings in the jetting head are coplanar and positioned at an angle to the longitudinal axis of the tool. The angle is chosen so that the plane of the jet openings is perpendicular to the axis of least principal stress in the formation being fractured. The tool must be custom-made for each job, since the entire jet head is angled into the tool.
U.S. Pat. No. 6,832,654 B2 by Ravensbergen et al., “Bottom Hole Assembly”, discloses a bottom hole assembly (BHA) in the form of a straddle packer for positioning an abrasive jet perforating tool. The BHA includes a timing mechanism to keep dump ports open to flush underdisplaced fluids from the BHA, a release tool in case the BHA gets stuck in the wellbore, and a mechanical collar locator.
U.S. Pat. No. 7,159,660 B2 by Justus, “Hydrajet Perforating and Fracturing Tool” discloses an abrasive jet perforating and fracturing tool. The tool comprises both abrasive jet ports and fracturing ports having larger apertures than the jet ports. The fracturing ports are used to eject fracturing fluid into the formation at a faster rate than possible through the jet ports. The tool further comprises a rotating sleeve, turned by a power unit, with apertures that align or misalign with the jet ports and control ports to control flow through the ports.
A common concern for downhole tools in general, and abrasive jet perforating tools in particular, is the potential for getting the tool lodged or caught in the hole. As the abrasive jet perforating process begins, sand laden fluid is pumped through the tool at high pressure to cut through the casing and extend a cavity into the reservoir. As the fluid jet is cutting through the steel casing, all of the sand that passes through the orifice remains in the annulus of the casing. While some of this sand falls toward the bottom of the hole, some of the sand is pushed upward by the turbulent fluid action of the jet. If the fluid conditions (depending upon the viscosity of the fluid and the rate of fluid flow) are favorable, then the sand could return to the surface in the fluid flow, or, alternatively, the sand could travel a distance upward, lose velocity, and then fall back toward the bottom of the hole, settling wherever it can. Once the abrasive jet perforating tool has cut a hole in the casing, the sand particles enter the cavity that is being cut, but since the cavity is closed, most of the sand will return to the casing. The cuttings from the reservoir will also flow to the casing as the cavity is cut, creating more material in the annulus of the well. If the volume of the sand and formation cuttings deposited on the tool is too great, the tool could become trapped in the well by the material settling on the bottom hole assembly.
An additional concern in openhole conditions (a well without a casing) is that large pieces of the formation might fall into the well bore as the abrasive jet cuts its path. With a cased reservoir, the perforation hole in the casing limits the particle size of the cutting that can be flushed back into the annulus. In openhole wells, the particle size is not limited and, depending on the strength of the reservoir, large pieces of rock could break loose and fall into the wellbore, lodging in between the tool string and the wall of the well.
Thus, a need exists for a sand jet perforating tool and method of use that provides improvements to the sand jet perforating tool design that allow for improved performance, more cost effective operation, and increased security of the intellectual property.